Electrolytic recording sheets



Jan. 12, 1965 B. R. HARRIMAN ELEcTEoLYTIc RECORDING SHEETS NEX@ 36 ww QN United States Patent O 3,165,425@ ELECTRLYTEC RECRDENG SHEETS Benjamin R. Harriman, St. Paul, Minn., assigner to Minnesota Mining and Manufacturing Company, St.. Pani, Minn., a corporation of @sian/are Fiied Sept. 22, 196i, Ser. No. 140,032 Ciaims. (Qi. 20d-d) This invention relates to new and useful sheets for the reproduction of visible images. in one aspect, this invention relates to improved image recording sheets which can be developed by electrolytic techniques. in another aspect, this invention relates to a method-for the preparation of such sheets and to a method yfor their use in image reproduction.

Photoconductive coatings and sheet constructions have been suggested for a variety of purposes, including the reproduction of light images. A recently developed image reproduction process involves electrolytically developing permanent and visible images on suitable photoconductive copysheets after exposure to actinic light images. This method, described more fully in United States patent application, Serial Number 575,070, filed March 30, 1955, now US. 3,010,883, includes the electrolysis of an electrolytic developer and particularly the electrodeposition of a metallic or other visibly distinct coating at the exposed photosensitive surface, usually by electrolytic reduction. No preliminary charging of the copysheet is required, and the copy produced needs no further heating or other processing to render the image permanent. However, the successful application of the electrolytic method has been Vfound to require, among other things, that the sensitive sheet be strongly photoconductive and have certain other conductivity requirements rather than merely the capacity to hold and dissipate an electrostatic charge. These copysheets, disclosed in greater detail in United States patent application, Serial Number 692,529, filed Gctober 28, 1957, now U.S. 3,010,884, generally comprise a photoconductive layer of zinc oxide particles in an insulative organic resinous binder superimposed on a contiguous electrically conductive layer, the combined layers having a conductivity of at least about 10-7 mho/cm. on exposure to 1300 foot candles of incident light from a tungsten source at 3100 K. (ie. light conductivity) and a dark conductivity not greater than about one-twentieth of the light conductivity. Copysheets employing photoconductive indium oxide and their preparation are described in United States patent application Serial Number 848,219, tiled October 23, 1959.

ln attempting to improve the quality of such photoconductive copysheets, various means for increasing the image density and contrast, using electrolytically reducible developer materials, such as silver salts, have been investigated. Heretofore, the optical diffuse reflection Vdensity of a silver image has tended to reach a maximum of about one regardless of exposure or quantity of silver deposited in the image areas, and the average gradient of diffuse reiiection density vs. log exposure curve, .which is a measure of contrast, has been less than one. Such values are too low for printing average continuous tone photographic negatives. Furthermore, the density has been inadequate for many line copy and half-tone copy applications. Moreover, an increase in image density must be achieved without pinpointing (Le. electrolytic breakdown of the 4photoconductive coating, causing points of heavy developer deposit), without yellowing or discoloration of the sheet, and without loss of the other desirable properties of such copysheets, e.g. good definition, ease and speed of development.

It is therefore an object of this invention to provide an improved photoconductive copysheet.

3,ib5,453 Patented Jan. 12, i965 "ice Another object of this invention is to provide a photoconductive copysheet which can be electrolytically developed to produce a visible image having improved image density, greater image contrast and better smudge resistance.

Still another object of this invention is to provide an improved photoconductive copysheet which is resistant to pinpointing and to yellowing.

A further object of this invention is to provide a photoconductive copysheet of improved appearance.

Yet another object of this invention is to provide a process for preparing an improved photoconductive copysheet.

Another object of this invention is to provide a means for preparing photoconductive copysheets having different degrees of gloss.

Other objects and advantages will become apparent from the following disclosure and examples.

An improved photoconductive copysheet of this invention comprises a continuous electrically conductive layer having thereon a thin, continuous and contiguous layer of photoconductive particles in a Water-insoluble, insulative binder and, superimposed thereon, though not necessarily in direct contact therewith, a thin, continuous, glossy, adherent, transparent, cohesive, water permeable and water insoluble layer comprising a film forming silica. The silica must have film forming properties, as distinct from the silicas which dry from the aqueous sol to noncohesive, readily disruptable powder. The former siiicas are capable of forming a stable colloidal sol with a particle size in the 1 to 100 millimicron diameter range, preferably from about 10 to about 50 millimicrons.

Those silicas which are capable of producing the desired glossy, adherent, cohesive, water permeable layer are available as aqueous colloidal sols with up to about 50% solids in the above particle size range, which sols are of high purity and are substantially free of alkali metal cations, eg. sodium, potassium, etc. They may be prepared by the progressive growth in aqueous sodium silicate solution of silica particles, such silica forming on nuclei of silicio acid. The alkaline sol thereby produced is deionized to remove sodium ion and reduce the pH to below 5, preferably below 4, by passage through an ion exchange bed. Such procedure is described in United States Patent No. 2,244,325. An illustrative silica sol is Nalcoag 1034A, a deionized acid sol containing 34% solids by weight and a particle size range of 16 to 22 millimicrons (available from Naco Chemical Company, Chicago, Illinois). Other Nalcoag siiica sols, such as those identified as 1015, 1022, 1030, 1035 and 1050, are suitable after deionization. Silica sols sold by E. I. du Pont de Nemours and Co., Wilmington, Delaware, under the trade name Ludox may also be used after deion-ization. Chemically modiiied silicas, eg. esterified silicas, having the above properties may also be used. The pyrolytically produced silicas, though readily dispersible in water, are generally incapable of forming the desired glossy and continuous, cohesive film on the photoconductive copysheet surface. Additional ion exchange treatments of available film forming silica sols may be desired to remove traces of undesirable alkali metal cations, such as sodium ion, and hence to reduce the tendency of the top-coated photoconductive copysheet to pinpoint during electrolytic development. Por example, less than parts per million of sodium ion (calculated as NaOH) is preferred.

The various silica sols may be readily evaluated for suitability as film-forming silica sols and for other of the aforementioned properties by dip coating a ground, non-corrosive microscope slide into the silica sol, and removing and drying either with a hot air current or at ambient room conditions. The surfaces of the slide should be previously ground with a fine abrasive, i.e. a grit size in the 320 to 500 mesh range, The useful silica sols willdry Without blushing to a water insoluble film that reduces theY dispersivity and increases the transparency ofthe 'ground glassfsurface. Aqueous solutions of dyes such as methylene blue will .be strongly adsorbed on the coated surface and are not substantially removed by krinsing in Water. If the silica sol provides too thick a coating, as evidenced vby crazing or crackin-g of the lm, the sol is diluted with water to reduce the coating thickness and the test is repeated. In distinct contrast to the useful silica sols, other silica sols of a non-film forming nature evidence blushing upon drying andwill not dry to a coating having any significant transparency. Moreover, the non-film forming silica coatings are primarily particulate and can be rubbed off the surface` readily. Little or no dye, eg. methylene blue, will adhere to the coated area upon rinsing the dyed slide in water.

In contrast to the film 'forming silicas, organic film formers, such as polyvinyl alcohol, polyvinyl pyrrolidone, dimethyl hydrantoin formaldehyde, gelatin, etc.` have proven unsatisfactory and tend to produce such harmful effects as a significantly lower development rate, pin-` pointing, formation of gas bubbles under the topcoating 'during development, and serious residual developer stain or discoloration.

The glossy silica topcoatings may be prepared by dip, knife, bar, brush or roller coating of the photoconductive copysheet surface with an aqueous sol of the film forming silica, or, if desired, by such techniques as doctoring, gravure offset, etc. to insure uniform coating thickness. Other coating techniques, including those errployed in the photographic art, may also be used. Generally, any coating method used for coating a uniform ylayer of low viscosity material upon a non-absorbent substrate can be utilized. Surfactants, either cationic or nonionic, may be included in the aqueous silica sol to cnhance wetting characteristics and improve the uniformity of the resultant coating. When used, such surfactants are employed at low concentration to avoid fogging the photoconductive coating. surfactant concentrations are preferably below 0.06% based on silica Weight. Wetting eharcteristics may also be enhanced by including a wateramiscible organic solvent, up to'about 20% by volume, in the aqueous sol. Lower alkyl alcohols, Vsuch as isopropanol and methol, are particularly desirable for this purpose. Generally, the volatility of the water miscible organic solvent should be close to that of water to prevent orange pee caused by non-uniform solvent lloss during the drying of the silica topcoating.

Coating thickness is important in obtaining the improvements discussed herein. Although ythe coating effect the desired continuous film and gloss characteristics and t-he maximum rate at which crazing'of the film observable by the naked eye is produced, coating Weight of thek silicaon the photoconductiveV copysheet surface can be adjusted to obtain the desired balance of properties, such as maximum glosswith good Water permeability and fast-electrolytic development rate. From about 0.1 to aboutv 10, preferably from l to 6, grams of silica Yper square meter has proven satisfactory in most cases. With coating weights above about l grams of silica per square meter the electrolytic development rate becomes relatively slow.Y For lower gloss papers, the lower coating weights areernployed.

The transparent cohesive silica films of this linvention may also include various other additives, including the developer materials themselves (eg. zinc, silver and other metal salts, electrolytically reducible dyes), and a wide variety of other materials such as filter dyes, AlGOH, ammonium 'persulfate, ceric sulfate, HC1, HNO3, H3PO4, ZnCl2, ZnF2, Zn(NO3)2, Mg(NO3)2, acetamide, MgSCh urea,V acrylamide, quinone, acetic acid, thiourea, NN- methylene bis-acrylamide, ethylene diamine teu-acetic acid, ascorbic; acid, hydroquinone, phenols, morpholine, sucrose, carbowax, etc.V Up to about Weight percent,

preferably below about l0 Weight percent,`based on the Weight of film forming silica, of non-film forming silicas Vand colloidal boehmite with an average particle size below about 100 millimicrons mayl also be included in the silica sol formulation. Any additivel employed should not impair the transparency and otherof. the earlier mentioned useful properties Vof the topcoating. Ifapaper of lower gloss is desired, one may either apply a thinner coating of the film forming colloidal silica, asfnoted earlier, or incorporating into the silica. sol a non-film forming silica of larger particle size to provide a semi-gloss or matte surface, usually Vup to'about 20 Weight percent of the film forming silica. Y n' Although the glossy silica layers of the invention are usually coated directly onto the photoconductive surface,

Weights will vary depending on the degree of gloss desired and consequently on the physical nature of the photoconductive surface,the dry coating weight must be thick enough to provide a continuous glossy topcoating but thin enough to avoid any noticeable .excessive crazing or minute lcrack formation in the silica topeoating. Grazing is aggravated by uneven or excessively ra id drying of the wet silica coating, e.g. by elevated temperature drying, as is known in paint technology. Air drying at in an amount insufficient to adversely affect the gloss characteristics of the dry coating, usually up to about 25% by Weight of the silica. Withinrthe silica coating Weight range encompassed bythe minimum Weight to they may also be coated onto intermediate layers or materials which are themselves in contact with thel photoconductive surface. Thus, for example, various"photoconductive copysheets having a thin, light .transmissive indium oxide developer coating on the photoconductive layer, as described in United States patent application Serial Number 848,272, filed Octoberv 23, 1959, may be overcoated with the glossy silica layers of this invention. Moreover, various'otherphotoconductive copysheets are provided with avgelatin coating containing the desired developer materials, as exemplified in United States Serial Number 575,070,.now U.S. 3,0l0,883, and these .may also be overcoated with the glossy silica layers of this invention. As used herein, therefore, the term superimposed on includes bothdirect and indirect Contact Withthe underlying photoconductive layer.

After the silica coated photoconductive copysheets of this invention are developed, particularly when a silver' salt isincluded in the ydeveloper solution, a stabilization step maybe desired to prevent discoloration of the baci/- ground area by entrapped developer in .the silica overcoating. Treatment of the copysheet surface with any of the silver stabilizers employed in the photographic art, e.g. quaternary ammonium-halides (such as Triton 400), saccharine, ethylene thiourea, etc. may be-used for this purpose.. f

Photoconductive copysheet constructions which can be coated With these film forming silicas have been described above with'referenceto other United States patent applications. Useful highly photoconductive materials include zinc oxide, indium oxide, cadmium sulfide, etc. The photoconductive particulate materials are uniformlyk dispersed in an insulating binder matrix, e.g. a butadienestyrene copolymer, etc; and are usually coated onto an electrically conductive backing, eg. aluminum foil, polyester or other plasticfihn vapor coated with aluminum silver, etc. The backing need not necessarily be electrically conductive if such photoconductive copysheets are to be developed by electrostatic techniques. Sensitizing dyes, such as Phosphine R (CI. 788), Patent Blue (CI. 672), xylene cyanol (Cl. 715), etc., are usually included in the photoconductive layer to broaden the spectral response of the photoconductor.

The electrolytic image reproduction process, in which the copysheets of this invention may be used, has made possible the direct copying of microfilm reproduction of printed pages of books, papers and the like Within a short period of time from initial exposure to the light image to delivery of the completed print. The improved copysheets of this invention are valuable because of their smudge resistance and ability to provide prints or reproductions having significantly higher image density, greater contrast and good definition Without significant loss 'of such other properties as resistance to pinpointing and a relatively rapid development rate. Also, these improved photoconductive copysheets have made photoconductive copysheets suitable for printing continuous tone black and white negatives as Well as for making monochrome prints from continuous tone color negatives.

To measure the etiect of a film forming silica overcoating on image properties, samples of a photoconductive zinc oxide copysheet (4.6/1 Weight ratio of zinc oxide to 30/ 70 mol ratio copolymer of butadiene/ styrene on aluminum foil laminate backing) were overcoated with a 110 line gravure roll using the following film forming silica dispersion:

60 parts by weight lilm forming silica sol (Nalcoag 1034,

A sample of both the uncoated and the silica overcoated copysheets was then exposed through a -3 neutral density continuous Wedge to 300 foot candles of incident light. FIGURES 1-3 represent a plot of the diffuse reflection density vs. the log relative exposure for various developers, curves A and B being the uncoated and coated results respectively in each ligure.

In FIGURE 1 a ten second exposure Was used. Electrolytic development was accomplished by immersing the exposed sheets in a developer containing 0.66 wt. percent silver nitrate, 1.8 wt. percent ethylene thiourea, 1 Wt. percent glacial acetic acid, 5 wt. percent magnesium acetate Agreater light penetration and absorption.

with the uncoated copysheets which tend to reach a maximum diffuse reiection density of about 0.9 or less.

Photoconductive zinc oxide copysheets (4.6/1 Weight ratio ZnO to Pliolite S7 binder on aluminum laminate backing) Were developed with an aqueous solution containing silver salt and thiourea after an extended exposure to light (i.e. saturation exposure). By controlling development time, varying image densities were obtained up to the maximum value of about 0.9V diffuse reflection density, as shown in FIGURE 4, curve A. By overcoating these developed copysheets with a film forming silica sol (Nalcoag 1050, deionized, 3-4 grams per square meter dry coating weight) the optical densities, as measured by the diffuse reflection densities, Were increased up to about 0.2 unit, as shown in FIGURE 4, curve C.l This may be attributed -to the optical effect resulting from less ightV being reflected from a glossy surface and hence Other photoconductive zinc oxide copysheets were overcoated With the same silica sol at the same coating Weight level before exposure and development. After exposure and development in the same manner, the optical density Values were markedly higher, as shown in FIGURE 4, curve B. These results indicate that higher diiuse reflection density values cannot be attributedsolely to optical effect, although the actual mechanism is notfully known. To a deionized aqueous sol of film forming silica containing 28.5 parts by Weight of silica in 66.5 parts of Water (i.e. Na'lcoag 1034A) was added 4.75 parts of isopropanol and 0.25 part of concentrated sulfuric acid. This sol was then coated onto a photoconductive copysheet (zinc oxide in a binder matrix of butadiene-styrene copolymer on a backing of a polyethylene terephthalate vapor coated with aluminum) in a dry coating Weight of 3 to 4 grams per square meter. A highly glossy finish was produced which Was not removed with pressuresensitive cellophane tape in the standard test. This overcoated copysheet was exposed and developed With an aqueous silver developer (2% AgNO3 and 2% magnesium nitrate) to produce a dense, stable, high contrast image with excellent definition and good resistance to smudging and without pinpointing. Similar results were obtained with the use of methyl alcohol rather than isopropanol and zinc sulfate rather than sulfuric acid.

and 91.54 wt. percent distilled water, the sheets being con- Y nected as cathode, and electrolyzing for 9 seconds at an applied voltage of 10 volts. The total current on-time was of the total immersion time, since the electrolysis was cycled to reduce polarization effects.

In FIGURE 2 a ten second exposure was also used. Electrolytic development was achieved by immersing the exposed sheets in a developer containing 20l parts by Weight NiCl26H2O, 4 parts ammonium chloride, 4 parts of 28% aqueous ammonium hydroxide, and 200 parts of distilled water (pH of 6.5), the sheets being connected as cathode, and electrolyzing continuously for 5 seconds at a voltage of 25 volts.

In FIGURE 3 a five second exposure was used. Electrolytic development Was accomplished by immersing the exposed sheets in a developer containing 2 wt. percent silver nitrate, 2 Wt. percent magnesium nitrate and 96% distilled Water, the sheets being connected as cathode, and electrolyzing for 0.5 second at 60 volts. The total current on-time was 10% of the total immersion time, since the electrolysis was cycled to reduce polarization eiiects.

From the curves of FIGURES 1 3-, the higher maximum density, steeper gradient and lower fog are evident, as compared to the uncoated copysheets. Moreover, another significant property of the silica overcoated copysheets is their ability to improve both density and contrast with continued extended development, as contrasted As mentioned earlier, the film forming lsilica layer must be Water permeable. A simple test for water permeability may be conducted by coating a magnesium plate With the silica sol to be evaluated and drying the coating either with a hot air current or at ambient room conditions to form a continuous coating on the plate. A few drops of dilute aqueous hydrochloric acid solution is then placed on the coated surface. After a few minutes the coating is physically removed and any etching of the metallic zinc substrate is noted. Water permeability is indicated by such etching.

Various other embodiments and modifications will be apparent to those skilled in the art from the foregoingl disclosure Without departing from the spirit and scope of this invention.

I claim:

1. In a photoconductive copysheet capable of electrolytic development the improvement which comprises, superimposed on the photoconductive surface thereof, a continuous, glossy, transparent, cohesive and water permeable layer comprising a film forming silica having an average particle size range from about 1 to about 100 millimicrons and a sulfate, said layer having a silica coating weight of from about 0.1 to about 10 grams per square meter and a sulfate content up to about 25 percent by Weight of said silica, said layer being essentially free of alkali metal ions.

2. In a photoconductive copysheet capable of electrolytic development the improvement which comprises, superimposed on the photoconductive surface thereof, a continuous, glossy, transparent cohesive and Water perphotoconductive coating.`

guesses meable layer'comprisingalm forming silica having an average particle size range from about 1 tok about 100 millimicrons and a sulfate, said layer having a silica coating weight of about l to about 6 per square meter and a sulfate content up to about 25 percent by Weight ofsad silica, said layer being essentially free of alkali metal ions.

' 3. A process yfor the preparation of improved photoconductive copysheets capable of electrolytic development which comprises applying over the photoconductive coating thereof a lm Vforming silica sol having an average silica particlersize of rornabout 1 to about 100 millirnicrons, said/silicasol being essentially free ofalkalir Vmetal ions, lin an amount su'flicient to provide a glossy dry 'coating having esesntially no crazing observable by the naked eye and having a coating weight of from about 0.1 to about 10 grams per square meter, and drying said coating vto produce a continuous, glossy, transparent, co-

hesive and Water permeable vlayer superimposedonrsaicl 4. The process of claim 3 in which .said silicafsol is of a sulfate, based on vthe weight ofv said silica.

5. "The process of claim 3 in which said silica sol is anV aqueous Ysol containing up to about 2() percent by Volume of a Water miscible organic solvent. p

6. The process of claim 3 in which -said silica solisan YaqueousV sol containing up to about 2O percent by volume of a Water miscible lower alkyl alcohol. Y

7. A photoconductive copysheet capable of i' `an aqueoussol containing up to about 25' weight percentV electrolytic A Vdevelopment which comprises a continuous electrically fgrarnsfper square meter. Y

development which comprises a continuous electrically Y conductive layer having thereon a thin, continuous .and

contiguous layer of photoc'onductive particles" in a Waterinsoluble insulative binder and, superimposedk thereon, a thin, continuous, glossy,` adherent,y transparent, cohesive, Water permeable and Waterinsoluble"layercomprising a lrnr forming silica Yin a `coating WeightV offfrom about 0.1

to about l0 grams per square meter, said Vlayer'rbeingV essentiallyffree of alkali metal ions Y Y 9. The copysheet of clainrS in whichthe coating Weight of said lrn'forrningysilica is from about 1V to about 6 10. "The copysheet of claim 8 in Which'said iilin forming silica layer is essentially free of Grazing Yobservable by the naked eye. Y Y

Y `)References Cited in the'le this Apatent i UNTTED STATES PATENTS Bird Janes, 1941 2,886,434 Owens May 12,V 1959 v2,901,349

Schatert et al Aug.V 25. 1959 

7. A PHOTOCONDUCTIVE COPYSHEET CAPABLE OF ELECTROLYTIC DEVELOPMENT WHICH COMPRISES A CONTINUOUS ELECTRICALLY CONDUCTIVE LAYER HAVING THEREON A THIN, CONTINOUS AND CONTGUOUS LAYER OF PHOTOCONDUCTIVE PARTICLES IN A WATERINSOLUBLE INSULATIVE BINDER AND, SUPERIMPOSED THEREON, A THIN, CONTINUOUS, GLOSSY, ADHERENT, TRANSPARENT, COHESIVE, WATER PERMEABLE AND WTER INSOLUBLE LAYER COMPRISING A FILM FORMING SILICA, SAID LAYER BEING ESSENTIALLY FREE OF ALKALI METAL IONS. 